```ts
/** The factorial of n. */
export function factorial(n: number): number {
  if (!Number.isInteger(n)) {
    throw new Error('Argument must be an integer');
  }
  if (n < 0) {
    throw new Error('Argument must not be negative');
  }
  return n === 0 ? 1 : n * factorial(n - 1);
}
```